Gas turbines generally include a compressor, one or more combustors, a fuel injection system and a turbine. Typically, the compressor pressurizes inlet air which is then turned in direction or reverse flowed to the combustors where it is used to cool the combustor and also to provide air to the combustion process. In a multi-combustor turbine, the combustors are located about the periphery of the gas turbine, and a transition duct connects the outlet end of each combustor with the inlet end of the turbine to deliver the hot products of the combustion process to the turbine.
In an effort to reduce the amount of NOx in the exhaust gas of a gas turbine, inventors Wilkes and Hilt devised the dual stage, dual mode combustor which is shown in U.S. Pat. No. 4,292,801 issued Oct. 6, 1981 to the assignee of the present invention. In this aforementioned patent, it is disclosed that the amount of exhaust NOx can be greatly reduced, as compared with a conventional single stage, single fuel nozzle combustor, if two combustion chambers are established in the combustor such that under conditions of normal operating load, the upstream or primary combustion chamber serves as a premix chamber, with actual combustion occurring in the downstream or secondary combustion chamber. Under this normal operating condition, there is no flame in the primary chamber (resulting in a decrease in the formation of NOx), and the secondary or center nozzle provides the flame source for combustion in the secondary combustor. The specific configuration of the patented invention includes an annular array of primary nozzles within each combustor, each of which nozzles discharges into the primary combustion chamber, and a central secondary nozzle which discharges into the secondary combustion chamber. These nozzles may all be described as diffusion nozzles in that each nozzle has an axial fuel delivery pipe surrounded at its discharge end by an air swirler which provides air for fuel nozzle discharge orifices.
In U.S. Pat. No. 4,982,570, there is disclosed a dual stage, dual mode combustor which utilizes a combined diffusion/premix nozzle as the centrally located secondary nozzle. In operation, a relatively small amount of fuel is used to sustain a diffusion pilot whereas a premix section of the nozzle provides additional fuel for ignition of the main fuel supply from the upstream primary nozzles directed into the primary combustion chamber.
In a subsequent development, a secondary nozzle air swirler previously located in the secondary combustion chamber downstream of the diffusion and premix nozzle orifices (at the boundary of the secondary flame zone), was relocated to a position upstream of the premix nozzle orifices in order to eliminate any direct contact with the flame in the combustor. This development is disclosed in the above identified co-pending '246 application.
Perhaps the most important attribute of a dry low NOx combustor is its ability to premix fuel and air before burning. In addition to good premixing quality, the combustor must be able to operate in a stable manner over a wide range of gas turbine cycle conditions. The problems addressed by this invention relate to the degree of premixing prior to burning, and the maintenance of stability throughout the premixed operating range.